Cause

Incorrect switch configuration.

As shown in the following figure, two switches are connected using two ports on each, and the MAC address table for Switch A will sometimes record the switch as accessed on port A1 (VLAN 1) and at other times as accessed on port B1 (VLAN 2).

Procedure
  1. PC A sends an IP packet to PC B.
  2. The packet enters VLAN 1 in the switch with the MAC address of the switch in the destination field. Because the switch has not yet learned this MAC address, it does not find the address in its address table and floods the packet out all ports, including the VLAN 1 link (port A1) to the switch. The switch then routes the packet through the VLAN 2 link to the switch, which forwards the packet on to PC B. Because the switch received the packet from the switch on VLAN 2 (port B1), the switch's single forwarding database records the switch as being on port B1 (VLAN 2).
  3. PC A now sends a second packet to PC B. The packet again enters VLAN 1 in the switch with the MAC address of the switch in the destination field. However, this time the switch's single forwarding database indicates that the switch is on port B1 (VLAN 2) and the switch drops the packet instead of forwarding it.
  4. Later, the switch transmits a packet to the switch through the VLAN 1 link and the switch updates its address table to show that the switch is on port A1 (VLAN 1) instead of port B1 (VLAN 2). Thus, the switch's information on the location of the switch changes over time, and the switch discards some packets directed through it for the switch. This causes poor performance and the appearance of an intermittent or broken link.

Invalid forwarding configuration